students = {
    "Alice": {"age": 20, "grades": {"math": 90, "english": 85}},
    "Bob": {"age": 21, "grades": {"math": 75, "english": 92}}
}

# 访问嵌套值
print(students["Alice"]["grades"]["math"])  # 输出：90

# 修改嵌套值
students["Bob"]["grades"]["english"] = 95